In the old world, we had two ways to bind a Widget tree to a
RenderObject node, one way for RenderView and one mostly untested way
for other cases (it's only tested by the spinning_mixed.dart demo). For
fn3, I made these the same code path.
This patch also introduces GlobalKey, though the GlobalKey logic isn't
hooked in yet.
This is Hello World in the new world:
```dart
import 'package:sky/src/fn3.dart';
void main() {
runApp(new Text('Hello World!'));
}
```
- I extracted the BuildScheduler into a separate binding.dart file.
- Various changes to expose private members that are needed by
binding.dart.
- Registering the render objects for event dispatch.
- Convert the tests to use the new binding mechanism.
This doesn't yet have a RenderView or event handling.
- adds dartdocs
- replaces config setter with didUpdateConfig() so that you can't replace
the config outside of the system
- renames didUnmount() with destroy().
- rename Component to StatelessComponent, ComponentConfiguration to
StatefulComponent
- move debug dump to end of file
- renamed _holder to _element
